Luminescence and positron spectroscopies studies of tris(2,2,6,6-tetramethyl-3,5-heptanedionate) europium(III) and terbium(III) complexes containing 2-pyrrolidone as coligand

In this work is reported the synthesis, characterization, luminescent properties and positronium formation yields of europium(III) and terbium(III) complexes of stoichiometric formula Ln(dpm)3(2-pyr), where: Ln = Eu and Tb, dpm = 2,2,6,6-tetramethyl-3,5-heptanedionato (dipivaloylmethanate) ion, a β-diketonate ligand, and 2-pyr = 2-pyrrodilidone, a γ-lactam. The crystal structures were determined by single-crystal X-ray diffraction. The complexes crystallize in the space group Pī with one complex in the asymmetric unit. The Eu(III) and Tb(III) ions are seven-coordinated by six O atoms of three β-diketonate ligands, and one O atom of γ-lactam molecule. We present and discuss experimental intensity parameters of 4f-4f transitions in the Eu(III) complexes under UV excitation. The photoluminescent properties of the complexes depend on the energy positions of the ligand-to-metal charge transfer (LMCT) states. The temperature dependence of the Eu(III) 5D0 relaxation rate of the Eu(III) complexes are presented. Positronium formation in Ln(dpm)3 and Ln(dpm)3(2-pyr) (Ln = Eu and Tb) complexes were investigated. A correlation between the parameters of luminescence and positron annihilation spectroscopies has been observed. The results, which strongly evidence the participation of molecular excited states in the positronium formation, were then discussed in terms of the recently proposed Ps formation mechanism, named cybotactic correlated system kinetic mechanism (CCSKM).